The present invention relates to a method and device for irradiating an ion beam which conducts processing such as ion injection by irradiating the ion beam on a substrate. The present invention also relates to a method of manufacturing a semiconductor device by irradiating an ion beam onto a semiconductor substrate. More particularly, the present invention relates to a means for suppressing electric charging (charge-up) on a substrate surface in the case of ion beam irradiation.
FIG. 6 is a schematic side view showing an example of the conventional ion beam irradiation device. concerning a relation between a substrate 2 and an ion beam 14, refer to the plan view of FIG. 2.
This ion beam irradiation device conducts processing such as ion injection as follows. The spot-shaped ion beam 14, which has been drawn out from an ion source (not shown) and subjected to mass separation and acceleration, is irradiated onto the substrate 2 (for example, a semiconductor substrate) held by a holder 16 while it is reciprocatedly scanned in the direction X (for example, in the horizontal direction) by an electric field or magnetic field.
The substrate 2 and the holder 16 are mechanically reciprocatedly scanned in the direction Y (for example, in the vertical direction), which is substantially perpendicular to the direction X, by a holder drive unit 18. By the cooperation (hybrid scanning) of the above mechanical scanning with the scanning of the ion beam 14, the overall surface of the substrate 2 is uniformly irradiated with the ion beam.
On the upstream side of the substrate 2 and the holder 16, there is provided a plasma generating device 20. The plasma generating device 20 generates plasma 30 and supplies it to a portion close to the substrate 2 so that electric charging on the surface of the substrate 2 caused by irradiation of the ion beam 14 can be suppressed.
The plasma generating device 20 generates the plasma 30 in such a manner that gas (for example, xenon gas) introduced into a plasma generating container 22 is subjected to ionization by arc discharge conducted between a filament 26 for emitting thermoelectrons and the plasma generating container 22 which is also used as an anode. In the periphery of the plasma generating container 22, there is provided a magnetic coil 28 for generating, maintaining and transferring the plasma 30.
Filament voltage VF (for example, about 5 V) for heating the filament is impressed upon the filament 26 by a DC filament power source 32. Arc voltage VA (for example, about 10 V) is impressed upon between a positive electrode side end of the filament 26 and the plasma generating container 22 by a DC arc power source 34.
Further, in this example, there is provided a cylindrical reflector electrode 38 in such a manner that the cylindrical reflector electrode 38 surrounds a region from the plasma generating device 20 to a portion close to the upstream side of the substrate 20. A negative voltage, for example, a negative voltage of xe2x88x925 V is impressed upon this reflector electrode 38 by a DC reflector power source 40. Accordingly, the reflector electrode 38 pushes back electrons contained in the plasma 30, which have been emitted from the plasma generating device 20, to the center (that is, to a portion close to the route of the ion beam 14).
An ammeter 36 is connected between a connecting section 33, in which the filament power source 32 and the arc power source 34 are connected with each other, and the ground. It is possible for the ammeter 36 to measure plasma emitting current IP flowing between the plasma generating device 20 and the ground when the plasma 30 is emitted from the plasma generating device 20.
When the ion beam 14 is irradiated onto the substrate 2, a surface of the substrate 2 is positively charged by the positive electrical charge of the ion beam 14. Especially when the surface of the substrate 2 is made of insulating material, the surface of the substrate 2 tends to be electrically charged. When the plasma 30 is supplied to a portion close to the substrate 2 in the case of ion beam irradiation, electrons contained in the plasma 30 are drawn onto the substrate surface which is positively charged, so that the positive electric charge is neutralized. When the positive electric charge is neutralized, drawing of electrons onto the substrate 2 is automatically stopped. In this way, it becomes possible to suppress the substrate surface from being positively charged when the substrate surface is irradiated with the ion beam.
When the plasma generating device 20 is provided as described above, it becomes possible to somewhat suppress the substrate surface from being electrically charged when it is irradiated with the ion beam. However, it is difficult to completely suppress the substrate surface from being electrically charged when it is irradiated with the ion beam.
The reason is described as follows. Electrons in the plasma 30 emitted from the plasma generating device 20 have an energy distribution, which is called Maxwell-Boltzmann""s Distribution, for example, shown in FIG. 7. In this distribution, there is a peak in a portion of 2 to 3 eV; however, it contains electrons having energy, the intensity of which is much higher than that (for example, 10 to 20 eV). Therefore, the electrons, the intensity of which is much higher than that, are supplied onto the substrate 2, and the substrate 2 is negatively charged on the contrary. When the above electrons, the intensity of which is high, is supplied to the substrate 2, the charging voltage of the substrate surface is increased to a voltage corresponding to energy of the electrons concerned.
For the reasons described above, it was impossible to sufficiently suppress the substrate surface from being electrically charged by the prior art. For example, it was a limit to suppress the charging voltage of the substrate surface to be in a range from 10 to 12 V.
However, recently, there has been a strong demand of decreasing the charging voltage of the substrate surface by more suppressing the electrical charging of the substrate surface.
For example, in the case where a semiconductor device is manufactured by ion injection conducted by irradiating an ion beam, there is a demand that the charging voltage is suppressed to be a value not higher than 6 V in the case of ion injection in order to prevent the occurrence of electric breakdown because the structure of a semiconductor device is has become fine recently.
This will be described in detail referring to an example in which a semiconductor device 12 shown in FIG. 8 is manufactured. The semiconductor device is an example of FET (field effect transistor). More particularly, the semiconductor device is an example of MOSFET (MOS type field effect transistor). In the case where the semiconductor device 12 is manufactured, a semiconductor substrate (for example, the silicon substrate) 2 is used as the above substrate 2, a gate oxide film 4 and an oxide film 6 for separation are formed in a predetermined region on the surface of the semiconductor substrate, and a gate electrode 8 is formed on a surface of the gate oxide film 4.
When the semiconductor substrate 2 is irradiated with the ion beam 14, dopant ions (for example, ions of boron, phosphorus or arsenic) are injected. Due to the foregoing, two impurity injection layers 10 are formed in the surface layers of the semiconductor substrate 2 on both sides of the gate electrode 8 and the gate oxide film 4. For example, when ions of boron are injected as the dopant ions, these impurity injection layers 10 become the p-type, and when ions of phosphorus or arsenic are injected as the dopant ions, these impurity injection layers 10 become the n-type. For example, when the semiconductor substrate 2 is of the n-type, the pn-type joint is formed by injecting the p-type impurity layers 10, and one impurity injection layer 10 becomes a source and the other impurity injection layer becomes a drain. Therefore, the p-channel type MOSFET is formed as the semiconductor device 12. For example, when the semiconductor substrate 2 is of the p-type, the pn-type joint is formed by injecting the n-type impurity layers 10. Therefore, the n-channel type MOSFET is formed as the semiconductor device 12. A large number of semiconductor devices 12 described above are formed on the surface of the semiconductor substrate 2.
For the above reasons, when the ion beam 14 is irradiated, electric charge is accumulated on the surface of the gate electrode 8. When the charge voltage exceeds the withstanding voltage of the gate oxide film 4, electric breakdown of the gate oxide film 4 is caused, and the semiconductor device 12 becomes defective.
Recently, size L of one piece of the semiconductor device 12 becomes very small, that is, size L of one piece of the semiconductor device 12 is approximately 0.1 xcexcm. Accordingly, thickness of the gate oxide film 4 becomes small, that is, thickness of the gate oxide film 4 is approximately 4 nm, and its withstanding voltage is approximately 6 V. For the above reasons, it is necessary to suppress the charging voltage of the gate electrode 8 to be a value not higher than 6 V during the irradiation of the ion beam 14. It is difficult to accomplish this by the prior art described before.
It is an object of the present invention to suppress electric charging of a substrate surface to be low during the irradiation of ion beams.
The above-mentioned object can be achieved by an ion beam irradiation method, according to the present invention,. which suppresses electric charging on a substrate surface caused by ion beam irradiation by supplying plasma, which has been emitted from a plasma generating device, to a portion close to the substrate when ion beams are irradiated onto the substrate, the method comprising the steps of: keeping a ratio of IE/IB at a value not lower than 1.8; and keeping a ratio of II/IE in a range from a value not lower than 0.07 to not higher than 0.7, wherein IB is an electric current of the ion beam irradiated onto the substrate, II is an ion current expressing a quantity of ions in the plasma emitted from the plasma generating device, and IE is an electron current expressing a quantity of electrons in the plasma.
According to the invention, when the two ratios (IE/IB and II/IE) are kept in the above ranges, the positive electric charge on the substrate surface, which has been given by the ion beam irradiation, can be effectively neutralized by electrons in the plasma, and the negative electric charging caused by the electrons can be successfully neutralized by ions in the plasma. Accordingly, it is possible to reduce the electric charging of the substrate surface, so that the charging voltage on the substrate surface can be decreased.
The present inventors made various experiments. As a result, they found the following.
In order to effectively suppress electrical charging of a substrate surface which is caused in the process of ion beam irradiation, it is necessary to increase a quantity of electrons contained in plasma emitted from a plasma generating device more than a quantity of ion beams irradiated onto the substrate.
Even if electrons having energy, the intensity of which is high as described above, are contained in the plasma emitted from the plasma generating device, when a ratio of ions (positive ions) contained in the plasma is appropriately increased, the negative electric charge on the substrate surface can be successfully neutralized. Thereforef the practical charging voltage on the substrate surface can be reduced.
When the above two conditions are totalized, the following can be concluded. When the ion beam is irradiated onto the substrate, the above ratio IE/IB is kept at a value not lower than 1.8 and the above ratio II/IE is kept in a range from a value not lower than 0.07 to a value not higher than 0.7, the electric charge on the substrate surface can be reduced. Due to the foregoing, it is possible to suppress the electric charging voltage on the substrate surface to be a value not higher than 6 V.
When the above ratio IE/IB is lower than 1.8, a quantity of electrons supplied onto the substrate is so small that the substrate surface is positively charged, which is not preferable.
When the above ratio II/IE is lower than 0.07, a quantity of electrons supplied onto the substrate is so large that the substrate surface is negatively charged. On the contrary, when the above ratio II/IE is higher than 0.7, a quantity of ions supplied onto the substrate is so large that the substrate surface is positively charged. Accordingly, either of them is not preferable.
The above-mentioned object can be also achieved by an ion beam irradiation device for conducting processing on a substrate by irradiating an ion beam onto the substrate, according to the present invention, which suppresses electric charging on a substrate surface caused by ion beam irradiation by supplying plasma, which has been emitted from a plasma generating device, to a portion close to the substrate, the device comprising a control unit for keeping a ratio of IE/IB at a value not lower than 1.8 and also keeping a ratio of II/IE in a range from a value not lower than 0.07 to not higher than 0.7, wherein IB is an electric current of the ion beam irradiated onto the substrate, II is an ion current expressing a quantity of ions in the plasma emitted from the plasma generating device, and IE is an electron current expressing a quantity of electrons in the plasma.
According to the invention, it becomes possible to save labor necessary for operating the device and also it becomes possible to automatize the operation.
Further, the above-mentioned object can be achieved by a method of manufacturing a semiconductor device, according to the present invention, which suppresses electric charging on a semiconductor substrate surface caused by ion beam irradiation by supplying plasma, which has been emitted from a plasma generating device, to a portion close to the semiconductor substrate when ion beams are irradiated onto the semiconductor substrate so as to manufacture the semiconductor device, the method comprising the steps of: keeping a ratio of IE/IB at a value not lower than 1.8; and keeping a ratio of II/IE a range from a value not lower than 0.07 to not higher than 0.7, wherein IB is an electric current of the ion beam irradiated onto the substrate, II is an ion current expressing a quantity of ions in the plasma emitted from the plasma generating device, and IE is an electron current expressing a quantity of electrons in the plasma.
According to the invention, it is possible to reduce the electric charging of the substrate surface, so that the charging voltage on the substrate surface can be decreased when the semiconductor device is manufactured. Therefore, it becomes possible to prevent the occurrence of electric breakdown in the process of ion beam irradiation, and the yield of the semiconductor device can be enhanced in the process of manufacturing the semiconductor device. This method is capable of complying with the reduction of the size of the semiconductor device.